"If confirmed, this is another example of the extreme brutality of the North Korean regime. We are following developments in North Korea closely and consulting with our allies and partners in the region," it said in a statement.

China, North Korea's main economic backer, said the execution was an "internal matter" for North Korea.

However, the BBC's Martin Patience in Beijing says the bland statement is likely to mask deep concern and will raise questions as to how much influence China actually has over the nuclear-armed state.

Mr Chang - married to the elder Kim's sister - had held senior posts in the ruling party and was vice-chairman of the powerful National Defence Commission, the North's top military body.

He was frequently pictured alongside his nephew and seen by some observers as the power behind the throne.

But in early December, it emerged that he had been removed from his senior military position and that two of his aides had been executed.

Then on Monday, KCNA broadcast footage of him being removed from a party session by uniformed guards.

In a long report on Friday, KCNA said: "Chang dreamed such a foolish dream that once he seizes power by a base method, his despicable true colours as 'reformist' known to the outside world would help his 'new government' get 'recognised' by foreign countries in a short span of time."

It also said Mr Chang:

Attempted to "overthrow the state"

Transformed his department into "a 'little kingdom'" and attempted to "trigger off discontent" within the army to mobilise a coup

Took control of the "major economic fields of the country" and "schemed to drive the economy of the country and people's living into an uncontrollable catastrophe"

Committed corruption by transferring construction units to his contacts

Committed irregularities related to a joint economic zone set up with China

Was responsible for unpopular currency reforms in 2009

Under questioning Mr Chang is alleged to have confessed: "I was going to stage the coup by using high-ranking army officers who had close ties with me or by mobilising armed forces under the control of my confidantes.

Analysis

By Lucy WilliamsonBBC News, Seoul

This purge is striking for its speed and publicity. The crimes Chang Song-thaek was accused of - disloyalty and insurrection - are the most serious North Korea can muster.

His proximity to power and his positions of responsibility had, state media said, led him to see himself as equal to the country's young leader, and seek to topple him.

Chang Song-thaek had a lot of power over economic decisions, and regularly handled relations with China. Some believe that Chang's admiration for China's growth model led to policy differences within the regime.

But others say this is a battle for control of the North Korean state; that its young leader, Kim Jong-un, had outgrown his uncle's protection, and saw him instead as a rival and potential threat.

Chang Song-thaek has already been edited out of official documentaries: his story rewritten by the country's powerful propaganda machine. His dramatic fall from grace, designed to hammer the message home: that no one - not family members, not North Korea's most senior figures - is beyond retribution.

But the worry remains: how much does this story reveal about instability at the heart of the North Korean regime?

"I didn't fix the definite time for the coup. But it was my intention to concentrate my department and all economic organs on the cabinet and become premier when the economy goes totally bankrupt and the state is on the verge of collapse."

There was no immediate word about the whereabouts of Mr Chang's wife, Kim Kyung Hee.

Analysts say Mr Chang's fall from grace could be seen as the latest in a series of carefully calibrated moves to demonstrate Kim Jong-un's authority and an assertion of his independence.

In August 2012, Mr Chang made a high profile trip to China, where he met then-President Hu Jintao. The two sides later signed a raft of economic deals, including the development of two special economic zones: Rason, on North Korea's east coast, and Hwanggumphyong, on the border with China.

The BBC's Lucy Williamson in Seoul says one theory about Mr Chang's demise is that his work with China had led him to admire some of Beijing's economic reforms.

But it is more likely that he presented a perceived threat to his nephew's authority, she says.

Professor Lee Jung-hoon, from South Korea's Yonsei University, told the BBC that the move showed that North Korea was "very unstable".

"[For Kim Jong-un] to go to the extent to actually purge him and execute him says a lot about the state of things in that country," he said.

Media playback is unsupported on your device

Media captionState TV in North Korea announced the execution of a "wicked political careerist"

As news of the purge emerged earlier this week, South Korean President Park Geun-hye warned the North was "carrying out a reign of terror" to reinforce Mr Kim's position.

On Friday, South Korea's military said it had tightened surveillance on Pyongyang, news agency Yonhap reported.

Meanwhile, Japan's top government spokesman Yoshihide Suga said: "We will calmly monitor the situation while communicating with other countries and collect relevant information."

Kim Jong-il (d)

Kim Jong-il was one of the most secretive leaders in the world.Tales from dissidents and past aides created an image of an irrational, power-hungry man who allowed his people to starve while he enjoyed dancing girls and cognac.

But a different picture was painted by Sung Hae-rim, the sister of one of his former partners in her memoir, The Wisteria House.

She describes a devoted father and a sensitive, charismatic individual, although she admits even those closest to him were fearful of him.

North Korean media depicted him as a national hero, whose birth to the country's founder, Kim Il-sung, was marked by a double rainbow and a bright star.

Kim Kyung-hee

The youngest sister of the late Kim Jong-il and the wife of the man formerly regarded as the second most powerful figure in North Korea, Chang Song-thaek.

She has held a wide range of important Workers' Party positions including being a member of the all-powerful Central Committee.

Her promotion to four-star general made Kim Kyung-hee the first North Korean woman ever to achieve such status.

Analysts say Kim Kyung-hee and her husband were seen as mentors for the new leader Kim Jong-un when he came to power in 2011. But news of her husband's execution in December 2013 suggests the most significant upheaval in North Korea's leadership since Mr Kim succeeded his father.

Chang Song-thaek (d)

Chang Song-thaek was married to Kim Kyung-hee, the younger sister of the late Kim Jong-il. When the inexperienced Kim Jong-un became the new leader in 2011, the couple were widely thought to be acting as his mentors.

In December 2013, the powerful uncle - who sat on the country's top military body - was denounced by the state-run news agency for corruption. Images were shown of him being removed from a Politburo meeting by uniformed guards. He was then executed.

Mr Chang's execution is the biggest upheaval in North Korea's leadership since Mr Kim succeeded his father.

Kim Jong-nam

Sung Hae-rang, the sister of Kim Jong-nam's deceased mother Sung Hae-rim, has written in her memoir that Kim Jong-il was extremely fond of Kim Jong-nam and was pained to be away from him. Like his half-brothers, Kim Jong-nam studied at an international school in Switzerland.

His chances of succession appeared to be ruined when, in 2001, Japanese officials caught him trying to sneak into Japan using a false passport. He told officials that he was planning to visit Tokyo Disneyland.

Some analysts argued that he may have been forgiven by his father, as there is precedent for the regime reinstating disgraced figures after a period of atonement. Confucian tradition also favours the oldest son.

But in a rare interview while on a trip to China last year, Kim Jong-nam said he had "no interest" in succeeding his father.

Kim Jong-chul

Kim Jong-chul, 29, studied at an international school in Switzerland. He works in the WKP propaganda department.

His mother, Ko Yong-hui, is said to have been the North Korean leader's favourite consort.

However, Kenji Fujimoto, the pseudonym of a Japanese sushi chef who spent 13 years cooking for Kim Jong-il, has written that the leader considered his second son "no good because he is like a little girl".

Ri Sol-ju

Ri Sol-ju was introduced as Kim Jong-un's wife in state media reports about the opening of an amusement park in July 2012.

Reports simply said he attended the event with his wife, "Comrade Ri Sol-ju".

Little more is known about Ri Sol-Ju, although there has been much speculation about her background since pictures first emerged of Kim Jong-un with an unidentified woman. There is a North Korean singer of the same name, but she is not now thought to be the same person.

Kim Han-sol

The grandson of Kim Jong-il and nephew of leader Kim Jong-un has said he wants to "make things better" for the people of his country.

Kim Han-sol, 17, spoke of his dreams of reunification of the two Koreas in an television interview in Bosnia, where he is studying. Kim Han-sol said he had never met his grandfather or uncle.

He described an isolated childhood spent mostly in Macau and China, after his birth in Pyongyang in 1995. In the future, he said he pictured himself going to university and then ''volunteering somewhere''.